Treatment of metal surfaces



Jan. 17, 1961 H. M. BENDLER EI'AL 2,968,555

TREATMENT OF METAL SURFACES Filed Jan. 13, 195

2 Sheets-Shoot 1 Jan. 17, 1961 H. M. BENDLER ETAL 2,958,555

TREATMENT OF METAL SURFACES Filed Jan. 15, 1958 2 Sheets-Sheet 2 INVENTORS ATTORNEY zssasss TREANT or METAL suaracns Harry M. handler, East Lansing, Carl E. Bleil, Clawson, Theodore W. Hertzog, Detroit, Mitchell A. La Boda, East Detroit, and Manuel Ben, Gah fink, Mich, assignors to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Jan. 13, 1958, Ser. No. 708,514

1 Claim. (CI. 96-33) This invention relates to the treatment of metal surfaces and is more particularly concerned with the treatment and finishing of a metal surface to provide a multiplicity of fine pits therein.

It is well known that finely ground or honed metal surfaces are very smooth, thus greatly reducing the rate of wear due to friction if properly lubricated. However,-

due to the smoothness of such surfaces, they do not hold or retain oil, and it is therefore ditficult to insure or maintain sufficient lubricant over the surface to avoid the objectionable results arising from insuflicient lubricatlon.

It has been previously proposed to reverse the current after a layer of metal has been deposited electrolytically on the surface of a machine part so that the part becomes the anode. This process results in the removal of a small amount of metal from the surface so as to form numerous small grooves, pits or depressions in the surface, such being adapted to retain oil under working conditions. It has also been proposed to place a shield, such as a perforated plastic liner, on the surface in order to selectively etch the surfaceat the pointsof perforation. In the former method, it was impossible to control the size, shape or pattern of the grooves and pits. The latter method was subject to bleeding under the liner so that the grooves and pits were irregular in shape. This bleeding also limited the size of the pits or depressions and the number that could be etched in the surface per unit area.

Accordingly, a principal object of the present invention is to provide a new and improved method for treating a metal surface to provide a multiplicity of fine pits, depressions or grooves of controlled number, shape and size (particularly depth). Another object of the invention is to provide an article having improved wearing characteristics as a result of a surface treatment providing a multiplicity of fine pits which hold lubricant.

These and other advantages are attained in accordance with this invention by a process for selectively etching a metal surface by coating the surface with a light-sensi-- tive photo-resist material and selectively exposing the resist material to light through a half-tone screen or negative having the desired pattern. The unexposed portions of the resist material are then removed, and the surface areas of the metal thereby exposed are subsequently etched with a suitable etchant. This process is particularly advantageous for forming a large number of Pat-testes Jan. 1?, test ice I manner include chromium, aluminum, steel, cast iron,

pits or depressions per unit area in any controlled arbinickel, copper, molybdenum, zirconium and titanium.

Other objects and advantages of the present invention will more fully appear from the following detailed description of specific embodiments thereof, reference being made to the accompanying drawings, in which:

Figure l is a somewhat diagrammatic cross-sectional view on an enlarged scale of a base metal member having an electrodeposited layer of metal thereon and a photo-resist material applied to the surface of the electrodeposit;

Figure 2 is a somewhat diagrammatic cross-sectional view on an enlarged scale of the composite article of Figure 1 with the photo-resist material removed and the surface etched to provide a multiplicity of fine pits therein;

Figure 3 is a somewhat diagrammatic plan view on an enlarged scale of a photo-resist mask over the electrodeposited layer shown in Figure 1;

Figure 4 is a somewhat diagrammatic plan view on an enlarged scale of an etched metal plate similar to the plate illustrated in Figure 2 with the mask removed, showing a multiplicity of pits in the plate;

Figure 5 is a photomicrograph of the resist mask shown in Figures 1 and 3 at a magnification of diameters;

Figure 6 is a photomicrograph of the etched metal plate shown in Figures 2 and 4 at a magnification of 40 diameters; and

Figure 7 is a photomicrograph of the etched metal plate of Figure 6 at a magnification of 230 diameters.

Referring more particularly to the drawings, in Figure 1 is shown a metal plate 11 which has been velectrodeposited on a base metal 12. A coating 13 of photoresist material is located on the surface of plate 11. This coating is applied by spraying or other suitable means after the plate has been thoroughly cleaned, the coating of photo-resist material subsequently having been exposed through a half-tone negative and the desired pattern developed therein after exposure to light. The resist material is allowed to dry after application and before exposure.

The photo-resist material may be any of the wellknown photo-resist materials which are resistant to the etchants that are used in this process. An example of a suitable material is a high copolymer such as lightsensitive, cinnamic acid ester of a member of the group consisting of cellulose and hydroxyl-containing cellulose derivatives, as a combined carrier and light-sensitive material and a sensitizer for the composition of a nitroaryl compound having a nucleus containing 6 to 10 carbon atoms, from 1 to 3 nitro groups being attached to the nucleus, the mono-nitro compounds being free of amino, hydroxyl, and forrnyl groups ortho to the nitro group, the nitroaryl compounds being free of an amino and a hydroxyl group in positions ortho to each other, and free of carboxyl and sulfo groups. A commercially available preparation which may be used in practicing the invention. is Kodak Photo Resist," manufactured and sold by Eastman Kodak Company of Rochester, New York. Various polyvinyl alcohols and polyvinyl chlorides also are suitable as polymerizable photo-resist materials. Of course, many other organic examples are light sensitive and will be polymerized by light. It is also possible to employ substances which are sensitive to heat, radio-activity, various catalysts, etc., but the lightsensitive materials appear to provide best results.

Figure 3 illustrates the coating 13 of photo-resist material after it has been exposed through a half-tone screen and developed. Prior to the exposing of the resist material, the half-tone negative is placed in intimate contact with the resist material in order to avoid distortion.

Numerous mechanical means may be used for insuring proper contact of the half-tone negative mm the resist material. For example, in the treatment of cylinder bores, fixtures have been employed in which a vacuum is applied to the film negative. If desired, a cellophane half-tone screen or negative having the desired pit-forming pattern and having a wax adhesive backing may be used in place of the photographic half-tone screen. This is easier to apply as it eliminates the need for vacuum to cause it to be held tightly to the surface of the part being processed.

The geometries of the negative may be varied depending upon the desired porosity and the lines per inch desired in the treated surface. Examples of geometry that have been used for treating cylinder bores are 300 lines per inch designed to give 25% porosity in the top portion of the bore, and 12.5% porosity in the bottom portion. It is calculated that a typical cylinder bore, after such a treatment, would contain approximately 53 million pits. Other desirable geometries include 30 lines per inch on die blocks, 32 lines per inch on bearings, 85 lines per inch on clutch plates and 133 lines per inch on cylinder bores. The porosity in each of these examples may be controlled by the individual hole size and the number of lines per inch. These examples are not to be considered limitations of the invention but merely illustrate its versatility.

After the negative has been assembled in contact with the resist material, it is then exposed to light for a suitable length of time. The type of light and the length of time needed for optimum results are, of course, varied to suit the type of resist material used.

Following exposure to light, the resist material is developed in order to provide the necessary geometric pattern in the photo-resist material. Only the portions of the resist material which have been blocked off from the light by the negative are dissolved by the developer, thereby forming a mask containing a plurality of perforations or holes 15 (see Figs. 1, 3 and 5) which expose selected areas of the metal plate beneath the resits material to subsequent etching.

Numerous commercial photo-resist materials are available, and the techniques involved will vary with the preparation. Kodak Photo- Resist Developer, which is currently manufactured and sold by Eastman Kodak Company of Rochester, New York, is an example of an appropriate developer. Trichloroethylene also can be employed. As is well known, light which contacts exposed areas of photo-resist material produces polymerization of this light-sensitive monomeric organic substance. This mechanism permits the developer, which is generally a mixture of organic solvents, to dissolve only the areas which have not been exposed to light and to leave the polymerized portions of the photo-resist material remaining on the surface of the metal to be subsequently etched. in other words, the developer dissolves the monomeric or partially polymerized, unsensitized areas, while the sensitized areas which have been exposed to light are insoluble. It is possible in some instances to reverse the mechanism and to employ a material which has its solubility increased upon exposure to light. Under these conditions, the portion of the photo-resist material which has been exposed to light is subsequently dissolved. The unsensitized areas are insoluble in the developer employed.

After the resist material has been sufiiciently developed it may be dyed, although dyeing is not necessary to the practice of the invention. The purpose of the dye is to indicate the degree of polymerization produced by the subsequent curing operation. in this manner it is readily possible to determine by visual inspection when the polymcrization reaction has progressed to a sufiicient extent. Kodak Photo-Resist Dye, which is manufactured and sold by Eastman Kodak Company of Rochester, New

preparation for developing- 4 York, is an example or a suitable commercially available dye.

The next step in the process involves curing the resist material. Some materials require baking at elevated temperatures, while others may be cured by allowing material to remain in contact with the atmosphere at room temperature. Either type of material may be efi ectively employed in practicing the present invention. Exposure to heat completes the polymerization of the resinous material, causing it to harden and tightly adhere to the metal surface. The portions of the layer of the photo-resist material surrounding the formed openings in the resits material and immediately adjacent the surface of the metal are normally most unlikely to be completely polymerized; and since adhesion is most important at these areas, the desirability of complete polymerization is obvious. It is also possible to use chemical catalysts, such as liquids or acid gases, to complete the polymerization of the resist material.

When the photo-resist material has been sufficiently cured, the metal surface to be treated and the adhering layer of resist material are immersed in a suitable etchant such as hydrochloric acid, ferric chloride, nitric acid, dilute sulfuric acid, acetic acid, sodium hydroxide and sodium carbonate solutions. HCl has been found to provide best results for etching chromium plated surfaces, while a, ferric chloride solution has proved to be very satisfactory for treating copper and aluminum. HNO can be advantageously employed for etching silver and cast iron. The etching may or may not be electrolytic, depending upon the etchant used and the metal involved. when the metal and photo'resist mask are immersed in the etchant, only the surfaces of the metal layer beneath the resist material which are exposed to the etchant through the holes 15 in the resist material are etched.

The depth of the geometrically arranged pits thus formed in the etched metal may vary over very wide ranges, depending on the particular application. Pits as deep as 0.015 or 0.020 inch can be conveniently formed, but generally pit depths between approximately 0.0002 inch and 0.002 inch are typical. The sizes of the pits which can be formed using any given negative is limited, of course, by the distance between the openings since undercutting ettects eventually cause adjacent pits to merge. Moreover, since acid etchants dissolve the metal at an equal rate in all directions, it is not possible to etch to a depth greater than about one-half the distance between adjacent pits. The length of time the surface must be etched depends upon the type of etchant employed, the metal being treated, the concentration of the etchant and the current density it the etch is electrolytic. When the etching operation has been completed, the etchant should be cleaned from the surface and the resist mask removed by a suitable solvent to prepare the surface for use. Figures 6 and 7 illustrate a metal surface in which the holes 14 have been etched and the resist mask removed.

The following specific example of processing in accordance with the present invention involves the treatment of a chromium plated cylinder bore. After the walls of the bore have been plated, ground and honed to finished dimensions, the chromium surface is cleaned of all foreign material. Any solution which does not detrimentally react with the chromium may be employed. Organic residues can be removed with appropriate solvents, such as benzene, methyl ethyl ketone, acetone, etc., after mechanical cleaning. The chromium surfaces also may be cathodically cleaned in an alkaline solution for about two minutes after a soaking period of approximately 15 minutes. 'A subsequent cold water rinse followed by pumicing for several minutes and a second cold water rinse also may be desirable for some applications. The chromium surface is then activated to increase its wettability by swabbing with 50% hydrochloric acid, a period of about 30 seconds being appropriate for chromium plated cylinder bores or liners.

The activated metal surface may then be again rinsed in cold water to remove the hydrochloric acid, after which it can be dried in hot air at a temperature of approximately 130 F. to 150 F. A light-sensitive material, such as the aformcntioned Kodak Photo Resist," is next sprayed on the chromium surface and dried in hot air at a temperature of about 110 F. to 120 F. A half-tone negative of arbitrary geometry is thereafter placed in intimate contact with the photo-resist material and held in position by means of a vacuum. The half-tone negative preferably used is designed to produce 25% porosity on the upper half of the cylinder and 12.5% porosity on the lower half by means of uniformly spaced pits of approximately 0.005 inch in diameter in the upper portion and about 0.0025 inch in diameter in the lower portion.

After the half-tone negative is in place, the photo-reresist material is exposed to light through the half-tone negative for about thirty minutes, using a cold cathode tube placed in the center of the bore. Alternatively, any other actinic light can be employed. The photo-resist material is then developed using a photo-resist developer, such as Kodak Photo-Resist Developer, with a develop ing time of approximately two to three minutes. The photo-resist material is next dyed, using black Photo- Resist Dye, rinsed in cold water for about five minutes, and forced air dried. In order to insure complete drying and adequate curing of the photo-resist material, it is baked for about one-half hour at a temperature of approximately 500" F. The masked chromium surface is then etched in concentrated hydrochloric acid solution, which preferably contains a suitable inhibitor to iron dissolution, for about fifteen to twenty-five minutes. The addition of about 0.5% Rodine 60 as an inhibitor has proved to be satisfactory. After etching, the chromium surface is rinsed in cold water, then hot water, and thereafter dried. The residual resist mask is removed by a suitable organic solvent such as methyl ethyl ketone, acetone or other ketones. v

The following chart sets forth results obtained by using the above process on the two chromium plated cylinder bores:

Pit Depth Porosity Pit Diameter Liner Top Bottom Top Bottom Top Bottom" In ch as 0. 001 0.001

Inches Percent 25. 4 25. 4

Percent Inches Inches 0.005 0.005

is preferably etched electrolytically at about six volts and one half ampere per square inch in a 40 to 42 Baum ferric chloride solution.

Any metal which can be etched, either chemically or electrolytically, and is capable of being coated with a light-sensitive resist material may be treated in accordance with the present invention. The pitted surfaces formed by this process effectively retain oil or other lubricant, thereby appreciably increasing the wear and score resistance of the treated parts.

It will be understood that the invention is not limited as specifically described or as shown in the drawings, but that certain modifications may be made within the scope of the claim without departing from the spirit of the invention.

We claim:

1 A process for selectively etching a multiplicity of small pits in a chromium coating of a plated bearing surface to provide in said coating a multiplicity of geometrically arranged, small pits capable of containing lubricant, said process comprising cleaning and removing foreign matter from said surface, activating said surface by treating the same with a hydrochloric acid solution, rinsing said solution from said surface, drying said surface, spraying said surface with a coating of a light-sensitive composition comprising a polymer capable of being rendered insoluble in developing solvent and etching solution by light exposure, heating said coating to dry the same and improve its adherence to the bearing surface, placing in intimate contact with said surface a half tone negative having a multiplicity of areas through which light can be passed, said negative having about 30 to 300 lines per linear inch, exposing said coating of said light-sensitive material to a source of light through said negative to thereby insolubilize said material at said areas, subsequently developing said light-sensitive material by dissolving the areas of said material which have been masked by said negative, heating said layer to substantially complete the insolubilizing of said coating, thereafter immersing said adhering developed layer in a substantially concentrated hydrochloric acid solution to etch a multiplicity of pits entirely within said chromium coating to a depth of about 0.0002 inch to 0.002 inch at the areas where the light-sensitive layers have been dissolved, then rinsing the hydrochloric acid solution from said surface and removing said layer from said chromium plated surface by dissolving said layer in a suitable organic solvent.

5 References fitted in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Steel, vol. 141, #21, 1957, pages 153-156.

UNITED STATES PATENT I OFFICE CERTIFICATION OF CURRECTION Patent No. 2,968,555 January 17, 1961 Harry M. Bendlef et a1.

It is hereby certified that error appears in the above numbered patent reqliring correction and that the said Letters Patent should read as corrected below;

Column 2, line 66, strike out "radio-activity,".

Signed and sealed this 20th day of June 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

